1. Field of the Invention
The present invention relates to a differential amplifier, and more particularly, to a differential amplifier capable of providing a larger range of output current.
2. Description of the Related Art
FIG. 1 schematically shows a circuit diagram of a conventional low drop-out (LDO) regulator. Referring to FIG. 1, the LDO regulator 100 comprises a differential amplifier 110, and an output terminal of the differential amplifier 110 is electrically coupled to a gate of a PMOS transistor PMA. A first source/drain terminal of the PMOS transistor PMA is grounded through the resistors R1 and R2 that are serially connected. In addition, the first source/drain terminal of the PMOS transistor PMA is grounded through an external capacitor Cext, and a second source/drain terminal of the PMOS transistor PMA is electrically coupled to a DC bias Vcc. Moreover, a parasitic capacitor C1 is between the output terminal of the differential amplifier 110 and the gate of the PMOS transistor PMA.
Referring to FIG. 1, the differential amplifier 110 further comprises a positive input terminal and a negative input terminal. Wherein, the positive input terminal of the differential amplifier 110 is grounded through an input voltage source Vr, and the negative terminal of the differential amplifier 110 is electrically coupled to a node where the resistors R1 and R2 are joined to form a negative feedback circuit.
In the LDO regulator 100, the external capacitor Cext causes a dominant pole of the frequency response when cooperated with the output impedances of the PMOS transistor PMA and the resistors R1 and R2, and causes a non-dominant pole when cooperated with the output impedance of the differential amplifier 110. In addition, in the frequency response of the LDO regulator 100, the dominant pole is occurred before the non-dominant pole.
The output impedance of the PMOS transistor PMA is inversely proportional to the load current IL of the PMOS transistor PMA. In other words, the output impedance of the PMOS transistor PMA decreases with the increase of the load current IL, one that pushes the dominant pole move toward to the high frequency zone, such that the dominant pole is very close to the non-dominant pole. Meanwhile, the phase margin of the LDO regulator 100 may be too small, thus the system stability is significantly impacted. Accordingly, in order not to impact the system stability, the variance of the current outputted from the LDO regulator 100 should not be too big. Consequently, the application of the LDO regulator 100 is extremely restricted.
U.S. Pat. No. 6,188,211 discloses “Current-Efficient Low-Drop-Out Voltage Regulator with Improved Load Regulation and Frequency Response” (Rinco-Mora, et al.). In this patent, a source follower circuit is disposed on the output terminal of the differential amplifier. With such design, the low drop-out regulator provided by U.S. Pat. No. 6,188,211 uses appropriate current bias to compensate the frequency response so as to increase the range of the output current. However, since it is required to dispose a source follower between the operational differential amplifier and the load in U.S. Pat. No. 6,188,211, although it resolves the problem of system instability under large current operation, it is not easy to operate under a small current environment.